1. Field of the Invention
The present invention generally relates to the production of a beam of particles and, more particularly, to the production of a beam of neutral particles having kinetic energies which are readily controllable over a wide range and of substantially arbitrary chemistry, especially suitable for the in-situ production of minute amounts of chemicals during semiconductor processing.
2. Description of the Prior Art
Devices using beams of charged particles have achieved wide utility in many well-known electronic devices. For example, the cathode-ray tube is a major component of television sets and sensitive measurement devices such as oscilloscopes which have been known for many years. The ease and speed with which a beam of electrons, each having the same small mass and the same charge, can be manipulated has also led to numerous other electronic devices such as electron beam commutators. For the same reason, the use of a deflectable electron beam has become of interest for direct writing in high resolution lithography processes and other semiconductor manufacturing applications. Accordingly, the field of electron beam optics has become highly developed and is capable of producing positional accuracy and resolution to a small fraction of a micron at the present state of the art.
Devices which produce beams of positively charged ions are also known in semiconductor manufacturing. Such devices are typically used for directing ions against a surface of a semiconductor structure (e.g. a wafer which may or may not have layers or other structures formed thereon) for purposes of implanting, depositing or etching a material. Ion beams are typically somewhat more difficult to manipulate than electron beams since the increased mass of ions of ions (relative to electrons) requires much higher levels of energy to manipulate and direct the beam (e.g. to extract from a plasma source, accelerate, deflect, focus, etc.). At the same time, the crystal structure of the semiconductor material is easily damaged even at relatively low velocities of relatively massive particles. For example, it is usually necessary to anneal a semiconductor material after an ion implantation operation to restore the crystal lattice structure and repair damage thereto caused by the kinetic energy of the particles used in the implantation process.
However, at low velocities, the mutual repulsion between like-charged particles is sufficient to cause substantial expansion of the beam and the directivity of the particles is easily lost since, at comparable energies, ion velocity is far less than electron velocities due to the much greater mass of ions. Therefore, there is a relatively small "window" of conditions where an ion beam can be maintained to perform a desired process on a semiconductor material. Therefore, ion beam processes have generally been limited to the use of a broad, unfocussed and relatively diffuse ion beam with relatively low ion flux.
Because of these limitations on ion beam devices imposed by the materials on which processes are to be performed, some highly complex and sophisticated devices have been developed in order to assist in ion confinement to a beam and to remove energy from the beam immediately prior to the target. U. S. Pat. Nos. 5,196,706 and 5,206,516 to John H. Keller et al. and assigned to the assignee of the present invention are exemplary of the complexity and sophistication of design which is required to expand the conditions within the ion beam, such as increased ion flux, in a manner consistent with low damage to the semiconductor material confinement of ions in a beam and to avoid destruction of the semiconductor lattice structure. U.S. Pat. No. 5,196,706, for example, uses a multi-element deceleration lens to converge the ion beam and then remove energy from it immediately adjacent to the target so that rapid expansion of the low-beam of low energy ions is tolerable even when the energy is reduced as low as 25 eV. U. S. Pat. No. 5,206,516, teaches maintaining of a plasma discharge over a substantial portion of the length of the ion beam to maintain the beam in a substantially space-charge neutralized condition to counteract mutual repulsion between ions and limit expansion of the ion beam.
The use of charged particles, itself, produces problems in the manufacture of semiconductor devices. As devices have become smaller and integration densities increased, breakdown voltages of insulation and isolation structures therein have, in many instances, been markedly reduced, often to much less than ten volts. For example, some integrated circuit device designs call for insulators of sub-micron thicknesses. At the same time, reduction of size of structures reduces the capacitance value of the insulative or isolation structures and relatively fewer charged particles are required to develop a electric field sufficient to break down insulator or isolation structures. Therefore, the tolerance of semiconductor structures for the charge carried by particles impinging on them during the manufacturing process has become quite limited and structures for dissipating such charges during manufacture are sometimes required; often complicating the design of the device.
While this latter problem could be avoided by performing processing with neutral particles, the charge of an ion or electron is the only property by which the particles can be manipulated and guided. Therefore, an ion must remain in a charged state until its trajectory can be established and the energy of the ion must be sufficient that its trajectory will remain substantially unchanged when neutralized by an electron. Even then, the trajectory may be altered and the flux of a neutral beam be severely depleted by collisions with other particles which may or may not have been neutralized and which may have trajectories which are not precisely parallel. Higher energies than are tolerable for semiconductor processing have been necessary to obtain such trajectories of neutral particles and, in any event, no device capable of providing any significant flux of neutral particles in a well-collimated beam has been developed to date. Further, once particles have been neutralized, there is no longer any property of the particles which can be exploited to remove energy from the particles in the beam to reduce particle energy to levels which are useful for semiconductor processing.
A further complicating factor is the quantum mechanical process by which neutralization of a particle by ion-electron (+/e-) recombination to reach a state where the electron is bound to the ion (as opposed to a space charge neutralized beam, referred to hereinafter as a pseudo-neutral beam, in which the ions and electrons are of substantially equal populations but are not bound together) takes place (e.g. by so-called photon mediated two-body radiative recombination, fragmentation two-body recombination, molecular modes excitation two-body recombination and other known or theoretical processes). While the underlying physical processes will be discussed in greater detail below, the allowed states of each of the recombination mechanisms are such that the probability of recombination is extremely small in free space due to both the low particle density in the high vacuum (about 5.times.10.sup.-5 Torr) and the distances available (about 2 mm) because of the mean free path of the particles and the simultaneous requirements of conservation of energy and momentum, causing the electron to assume an auto-ionization state after collision with an ion unless the center of mass frame (CMF) is zero or near-zero. Previously attempted processes such as a charge exchange process in which an ion beam is passed through a neutral medium are therefore largely ineffective to produce a significant population of neutral particles and the energies required to maintain even a depleted beam after passing through such a neutral medium are far in excess of the energies which are tolerable for semiconductor processing.
Nevertheless, there are other factors in semiconductor processing which cause continuing interest in the production of a beam of neutral particles. For example, while many highly effective lithographic, chemical and metallurgical processes are known for the formation of semiconductor device structures at relatively high yield, many of these processes have characteristics which are less than fully desirable. For example, the process of choice for preferentially etching SiO.sub.2 over silicon (as may be encountered at an interface oxide in processes which are generically referred to as interface tailoring) is an isotropic wet etch process using hydrofluoric acid (HF), referred to as a buffered hydrofluoric acid (BHF) dip, which, while highly effective for the purpose, has the disadvantages of involving unavoidable atmospheric exposure and the possibility of contamination of the etching solution with reaction products and other possible contaminants.
Beyond the reduction in manufacturing yield inherent in this process, such a wet etch requires far more of the material than can possibly take part in the chemical reactions of the etching process since a sufficient quantity of HF must be contained for immersion of the wafers and to provide good circulation of the etchant around the wafers. However, no alternatives to the volume requirements of a wet etch process, particularly using HF, have previously been available. Beyond the expense of providing and containing such a volume of etchant, HF is known to be a particularly strong acid with high chemical activity and highly destructive to biological tissue with which it comes in contact. Therefore, the mere existence of substantial quantities of such an etchant represents a potential biological and ecological hazard and further precautions for containment and protection of personnel during use of such materials presents a substantial further cost component in the manufacture of semiconductor integrated circuits.
In some other chemical and metallurgical processes, it has been possible to reduce the amounts of reactants and enhance delivery to the location of the semiconductor material by plasma processes and numerous such techniques are known such as sputtering and reactive ion etching (RIE). However, the development of plasma chemistry processes is largely an empirical art and the result of slight variations in plasma chemistry processes is not generally predictable, due to the nature of the plasma itself.
More specifically, a plasma is a state of material occupying a volume in which a significant percentage of the materials present are in the form of ionized species and electrons. Within a so-called quiescent plasma many dynamic processes are in substantial equilibrium. That is, in a steady-state or quiescent plasma, the production of ionized species equals the loss of ionized species by recombination. If ions or electrons are extracted from the plasma, they must be replaced by a current input and/or material flux.
Perhaps most important to an understanding of the dynamic nature of a plasma is that the major, inner, portion of a quiescent plasma is substantially space-charge neutralized with the net mutual repulsion between like-charged species balanced by mutual attraction between oppositely charged species. This means, for any charged particle which is well-separated from the boundary of the plasma but having a trajectory toward the boundary of the plasma, a force will be exerted on the plasma which tends to pull it back toward the plasma. Therefore, most of the volume of the plasma can be regarded as generally homogeneous.
However, within this population of charged species the electron mobility is far greater than that of the ions. Therefore the electrons tend to leave the ions at the boundary of the plasma, creating a so-called ambipolar potential. Therefore, there is a slightly greater population of ions near the boundary of the plasma and the repulsion forces therebetween also tends to accelerate the ions outwardly. This acceleration of ions will, however, decrease with increasing distance from the boundary of the plasma while electron acceleration increases. These conditions are effectively reversed when the boundary of the plasma is near a conductive surface which tends to return electrons to the plasma and to accelerate ions.
Therefore, either ions or electrons can escape from the plasma and it is seen that a quiescent plasma represents a source/sink dynamic balance and is neutral overall. However, because of the conditions at the boundary which assist the attainment of a dynamic balance, there is no good balance at any point within the plasma. Unfortunately, any surface at which chemical processes are conducted with the plasma is, by definition, such a boundary.
The conditions which prevail in the vicinity of the boundary have not been well-understood and the populations of charged or neutral species which may be present at the surface to take part in a lithographic, chemical or metallurgical process is largely unpredictable. For example, increasing a partial pressure of gas A in a plasma reactor will not necessarily lead to a predictable increase in reaction product AB. The plasma dynamics at the boundary of a plasma are further complicated by the fact that any contact of the plasma with a metallic or conductive surface causes the surface to be negative relative to the plasma and the plasma adjacent to the surface to be positive. This voltage differential is called the plasma potential. Therefore, at the boundary of a quiescent plasma, ions will be accelerated away from the plasma and the energies, trajectories and distributions of the ions at the target is not readily predictable. Further, if the plasma potential is increased to increase ion flux, as is generally desirable, energy of the ions may be increased to levels which can damage the crystal lattice structure of a semiconductor material.
It should also be recognized that any device which can form a directed stream of accelerated particles has potential applications far beyond semiconductor metallurgy due to well-established classical laws of motion. Many applications exist where transfer of kinetic energy between objects is desirable. Even the simple act of driving a nail into a piece of wood relies of the simple mechanism of impacting a fast-moving mass on another mass which is desired to be moved. Applications abound where the impact of highly accelerated minute particles to impart kinetic energy to objects would be desirable. Similarly, the process of acceleration of a particle produces a reaction thrust. However, either of these types of applications require production of a stream of particles outside the apparatus which produces the stream and no apparatus has heretofore been devised which is capable of producing a stream of highly accelerated particles (either charged or neutral) beyond the confines of the device itself.